1. Field of the Invention
The present invention relates to piezoelectric filters used for bandpass filters, and more specifically to a longitudinally coupled multi-mode piezoelectric filter that has a piezoelectric stiffened effect and uses coupling of different order modes.
2. Description of the Related Art
Conventionally, various piezoelectric filters have been used as bandpass filters. In a frequency range of between several MHz and several tens of MHz, dual-mode piezoelectric filters, which are small and inexpensive, are commonly used.
A dual-mode piezoelectric filter of this type is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 5-327401.
FIG. 20 is a sectional view of a conventional dual-mode piezoelectric filter that uses thickness extensional vibration.
A piezoelectric filter 201 includes a piezoelectric plate 202 that is polarized in the thickness direction. A pair of excitation electrodes 203 and 204 is provided on the upper surface of the piezoelectric plate 202, and a common excitation electrode 205 is provided on the lower surface of the piezoelectric plate 202 so as to oppose the excitation electrodes 203 and 204 via the piezoelectric plate 202.
In operation, an input voltage is applied between the excitation electrode 203 and the common excitation electrode 205 to excite the piezoelectric plate 202. This results in a symmetric mode shown in FIG. 21A and an asymmetric mode shown in FIG. 21B, both of which are coupled to define a filter band. The output is extracted between the excitation electrode 204 and the ground electrode 205.
Besides the dual-mode piezoelectric filter using the thickness extensional mode as described above, there is also a known dual-mode piezoelectric filter in which the piezoelectric plate 202 is polarized in the direction parallel to the upper surface thereof such that thickness shear vibration is utilized.
In the conventional piezoelectric filter 201, the coupling strength between the symmetric mode and the asymmetric mode is dependent upon the spacing between the excitation electrodes 203 and 204. Thus, the distance of between the excitation electrodes 203 and 204 determines the frequency difference between the symmetric mode and the asymmetric mode, thereby determining the passband.
Thus, to provide a filter having a wider band, the spacing must be reduced, and the coupling and the frequency difference between the excitation electrodes 203 and 204 must be increased.
The excitation electrodes 203 and 204 are typically formed of conductive paste using a screen-printing method. In screen-printing, however, the ability to minimize the spacing therebetween is limited. On the other hand, forming the excitation electrodes 203 and 204 by photolithography can reduce the spacing therebetween. This, however, leads to greatly increased cost.
There is also a problem with this arrangement in that an increased electrostatic capacity and decreased attenuation occur between the input and the output of the piezoelectric filter 201, even when the spacing between the excitation electrodes 203 and 204 is reduced.
To overcome the above-described problems, preferred embodiments of the present invention provide a longitudinally coupled multi-mode piezoelectric filter that achieves a wider band, greater out-of-band attenuation, and lower fabrication cost.
According to a first preferred embodiment of the present invention, a longitudinally coupled multi-mode piezoelectric filter is provided that utilizes piezoelectric stiffened effect. The longitudinally coupled multi-mode piezoelectric filter includes a multilayer piezoelectric element. The multilayer piezoelectric element has at least four excitation electrodes arranged substantially parallel to one another, and a plurality of piezoelectric layers each provided between two adjacent excitation electrodes and polarized in the direction that is substantially perpendicular to or substantially parallel to the excitation electrodes. The multilayer piezoelectric element has first and second end surfaces opposing each other in the direction that is substantially perpendicular to the excitation electrodes. The piezoelectric filter further includes a ground electrode, an input electrode, and an output electrode. The ground electrode is provided on the outer surface of the multilayer piezoelectric element and is electrically connected to a first group including at least two of the excitation electrodes, the at least two of the excitation electrodes being selectively arranged in the direction in which the piezoelectric layers are provided. The input electrode is provided on the outer surface of the multilayer piezoelectric element and is electrically connected to a second group including at least one of the excitation electrodes, the at least one of the excitation electrodes being arranged toward the first end surface and defined by excitation electrodes other than the excitation electrodes of the first group. The output electrode is provided on the outer surface of the multilayer piezoelectric element and is electrically connected to a third group including at least one of the excitation electrodes, the at least one of the excitation electrodes being disposed near the second end surface and being defined by excitation electrodes other than the excitation electrodes of the first group. Application of an input signal between the input electrode and the ground electrode causes vibration of different order modes to be excited and coupled, such that an output signal is extracted between the output electrode and the ground electrode.
According to the first preferred embodiment of the present invention, adjustment of the number of the piezoelectric layers enables the use of various different order modes, thereby producing a wider band filter characteristic.
On the other hand, in the conventional dual-mode piezoelectric filter, the spacing between the excitation electrodes provided on one surface of the piezoelectric plate determines the frequency difference of each mode, thus requiring increased accuracy when forming the excitation electrodes to produce a wider bandwidth. In contrast, according to various preferred embodiments of the present invention, a wider band is easily produced by simply selecting modes to be utilized.
Furthermore, in the conventional dual-mode piezoelectric filter, a reduction in spacing between the excitation electrodes provided on one surface of the piezoelectric plate to achieve a wider band causes an increase in the electrostatic capacity between the input and output, thereby reducing the attenuation. However, the multi-mode piezoelectric filters according to preferred embodiments of the present invention achieve a wider band without reducing the spacing between the excitation electrodes, thus producing a greatly increased attenuation.
Accordingly, a longitudinally coupled multi-mode piezoelectric filter having a wider band and greater attenuation is produced at a greatly reduced cost in preferred embodiments of the present invention.
In various preferred embodiments of the present invention, an nth harmonic and an (nxe2x88x921)th harmonic are used as the different order modes to produce a dual-mode piezoelectric filter according to preferred embodiments of the present invention. Alternatively, an nth harmonic, an (nxe2x88x921)th harmonic, and an (n+1)th harmonic are used as the different order modes to produce a triple-mode piezoelectric filter according to preferred embodiments of the present invention, thereby producing an even wider band.
According to a second preferred embodiment of the present invention, a longitudinally coupled multi-mode piezoelectric filter utilizes piezoelectric stiffened effect. The longitudinally coupled multi-mode piezoelectric filter includes a multilayer piezoelectric element. The multilayer piezoelectric element includes at least four excitation electrodes arranged substantially parallel to one another, and a plurality of piezoelectric layers each provided between two adjacent excitation electrodes and polarized in the direction that is substantially perpendicular to the excitation electrodes. The multilayer piezoelectric element has first and second end surfaces opposing each other in the direction that is substantially perpendicular to the excitation electrodes, and has first, second, third, and fourth side surfaces extending between the first and second end surfaces. The direction in which the side surfaces extend defines the longitudinal direction of the piezoelectric element. The longitudinally coupled multi-mode piezoelectric filter also includes a ground electrode, an input electrode, and an output electrode. The ground electrode is provided on the outer surface of the multilayer piezoelectric element and is electrically connected to a first group including at least two of the excitation electrodes, the at least two of the excitation electrodes being selectively arranged in the direction in which the piezoelectric layers are provided. The input electrode is provided on the outer surface of the multilayer piezoelectric element and is electrically connected to a second group including at least one of the excitation electrodes, the at least one of the excitation electrodes being arranged toward the first end surface and defined by excitation electrodes other than the excitation electrodes of the first group. The output electrode is provided on the outer surface of the multilayer piezoelectric element and is electrically connected to a third group including at least one of the excitation electrodes, the at least one of the excitation electrodes being disposed near the second end surface and defined by excitation electrodes other than the excitation electrodes of the first group. Application of input signal between the input electrode and the ground electrode causes excitation and coupling of vibration of different order longitudinal modes, such that an output signal is extracted between the output electrode and the ground electrode.
The longitudinally coupled multi-mode piezoelectric filter according to the second preferred embodiment produces a wider band and greater attenuation.
According to a third preferred embodiment of the present invention, a longitudinally coupled multi-mode piezoelectric filter utilizes piezoelectric stiffened effect. The longitudinally coupled multi-mode piezoelectric filter includes a multilayer piezoelectric element. The multilayer piezoelectric element includes at least four excitation electrodes arranged substantially parallel to one another, and a plurality of piezoelectric layers each provided between two adjacent excitation electrodes and polarized in the direction that is substantially perpendicular to the excitation electrodes. The multilayer piezoelectric element includes first and second end surfaces opposing each other in the direction that is substantially perpendicular to the excitation electrodes, and has first, second, third, and fourth side surfaces extending between the first and second end surfaces. The direction in which the side surfaces extend is the thickness direction of the piezoelectric element. The longitudinally coupled multi-mode piezoelectric filter also includes a ground electrode, an input electrode, and an output electrode. The ground electrode is provided on the outer surface of the multilayer piezoelectric element and is electrically connected to a first group including at least two of the excitation electrodes, the at least two of the excitation electrodes being selectively arranged in the direction in which the piezoelectric layers are provided. The input electrode is provided on the outer surface of the multilayer piezoelectric element and is electrically connected to a second group including at least one of the excitation electrodes, the at least one of the excitation electrodes being disposed near the first end surface and defined by excitation electrodes other than the excitation electrodes of the first group. The output electrode is provided on the outer surface of the multilayer piezoelectric element and is electrically connected to a third group including at least one of the excitation electrodes, the at least one of the excitation electrodes being disposed near the second end surface and defined by excitation electrodes other than the excitation electrodes belonging to the first group. Application of an input signal between the input electrode and the ground electrode causes excitation and coupling of vibration of different order thickness extensional modes, such that an output signal is extracted between the output electrode and the ground electrode.
The longitudinally coupled multi-mode piezoelectric filter according to the third preferred embodiment of the present invention also achieves a wider band and greater attenuation as in the first preferred embodiment of the present invention. Therefore, a piezoelectric filter that utilizes harmonics of a thickness extensional mode of vibration and achieves superior filter characteristics is produced.
According to a fourth preferred embodiment of the present invention, a longitudinally coupled multi-mode piezoelectric filter includes a multilayer piezoelectric element. The multilayer piezoelectric element includes at least four excitation electrodes arranged substantially parallel to one another, and a plurality of piezoelectric layers each provided between two adjacent excitation electrodes and polarized in the direction that is substantially parallel to the excitation electrodes. The multilayer piezoelectric element has first and second end surfaces opposing each other in the direction that is substantially perpendicular to the excitation electrodes, and has first, second, third, and fourth side surfaces extending between the first and second end surfaces. The direction in which the side surfaces extend is the thickness direction of the piezoelectric element. The longitudinally coupled multi-mode piezoelectric filter includes a ground electrode, an input electrode, and an output electrode. The ground electrode is provided on the outer surface of the multilayer piezoelectric element and is electrically connected to a first group including at least two of the excitation electrodes, the at least two of the excitation electrodes being selectively arranged in the direction in which the piezoelectric layers are provided. The input electrode is provided on the outer surface of the multilayer piezoelectric element and is electrically connected to a second group including at least one of the excitation electrodes, the at least one of the excitation electrodes being arranged toward the first end surface and defined by excitation electrodes other than the excitation electrodes of the first group. The output electrode is provided on the outer surface of the multilayer piezoelectric element and is electrically connected to a third group including at least one of the excitation electrodes, the at least one of the excitation electrodes being arranged toward the second end surface and defined by excitation electrodes other than the excitation electrodes of the first group. Application of an input signal between the input electrode and the ground electrode causes excitation and coupling of vibration of different order thickness shear modes, such that an output signal is extracted between the output electrode and the ground electrode.
The longitudinally coupled multi-mode piezoelectric filter according to the fourth preferred embodiment of the present invention also achieves a wider band and greater attenuation as in the first, second and third preferred embodiments of the present invention. Therefore, a longitudinally coupled multi-mode piezoelectric filter that utilizes harmonics of a thickness shear mode of vibration and which has superior filter characteristics is produced.
Preferably, the different order modes are an nth harmonic and an (nxe2x88x921)th harmonic, where n is an integer equal to or greater than 3.
Alternatively, the different order modes are preferably an nth harmonic, an (nxe2x88x921)th harmonic, and an (n+1)th harmonic, where n is an integer equal to or greater than 3. This arrangement produces an even wider band.
Preferably, at least one of the first group of the excitation electrodes is arranged between an excitation electrodes of the second group and an excitation electrode of the third group.
This arrangement greatly reduces electrostatic capacity between the input and output, thereby producing even greater attenuation.
A longitudinally coupled multi-mode piezoelectric filter according to various preferred embodiments of the present invention further includes reflection layers and holding portions. The reflection layers are respectively coupled to the first and second end surfaces of the multilayer piezoelectric element and are preferably made of material having a second acoustic impedance Z2 that is smaller than the acoustic impedance Z1 of piezoelectric material defining the piezoelectric layers of the multilayer piezoelectric element. The holding portions are respectively coupled to outer surfaces of the reflection layers and are preferably made of material having a third acoustic impedance Z3 that is greater than the second acoustic impedance Z2, the outer surfaces of the reflection layers oppose the first and second end surfaces to which the reflection layers are coupled.
In this case, vibration that has propagated from the multilayer piezoelectric element is reflected at the interfaces between the reflection layers and the holding portions. The holding portions, therefore, are used to mechanically hold the piezoelectric filter without affecting the characteristics thereof.